WASHINGTON — The Supreme Court has upheld a federal law that bars "material support" to foreign terrorist organizations, rejecting a free speech challenge from humanitarian aid groups.

The court ruled 6-3 today that the government may prohibit all forms of aid to designated terrorist groups, even if the support consists of training and advice about entirely peaceful and legal activities.

Material support intended even for benign purposes can help a terrorist group in other ways, Chief Justice John Roberts said in his majority opinion.

"Such support frees up other resources within the organization that may be put to violent ends," Roberts said.

Justice Stephen Breyer took the unusual step of reading his dissent aloud in the courtroom. Breyer said he rejects the majority's conclusion "that the Constitution permits the government to prosecute the plaintiffs criminally" for providing instruction and advice about the terror groups' lawful political objectives.

Justices Ruth Bader Ginsburg and Sonia Sotomayor joined the dissent.

The Obama administration said the "material support" law is one of its most important terror-fighting tools. It has been used about 150 times since Sept. 11, resulting in 75 convictions.

Most of those cases involved money and other substantial support for terror groups.